Various types of aromatic polyethers, including polyetherimides, polyethersulfones, polyetheretherketones, and polyetherketones, have become important as engineering resins by reason of their excellent properties. These polymers are generally prepared by the reaction of salts of dihydroxyaromatic hydrocarbons, such as bisphenol A disodium salt, with dihaloaromatic molecules such as bis(4-fluorophenyl)sulfone, bis(4-chlorophenyl)sulfone, the analogous ketones and bis(halophenyl)bisimides or bis(nitrophenyl)bisimides as illustrated by 1,3-bis[N-(4-chlorophthalimido)]benzene. Substantially equimolar proportions of the two reagents are normally required, with adjustments if desired for the presence of endcapping reagents such as 1-[N-(4-chlorophthalimido)]-3-(N-phthalimido)benzene.
One difficulty in the preparation of polyether polymers by these methods is the preparation of polymers of controlled and desired molecular weights. Since very nearly equimolar proportions of the principal reagents are used, it is generally very difficult to predict or control the molecular weight of the polyether polymer. For example, polyetherimides having a weight average molecular in a particular range may be desired by reason of their advantageous properties, but a given reaction may afford a product whose molecular weight is much higher or, more often, much lower. That product, being off specification, must be discarded, increasing the cost and size of the waste stream, with adverse consequences to the environment.
One approach to control the polymer molecular weight has been to employ a less than equimolar amount of the dihydroxy-substituted aromatic hydrocarbon to form an intermediate polymer having a low molecular weight and then adding a second portion of dihydroxy-substituted aromatic hydrocarbon sufficient to result in a polymer having the desired molecular weight. The amount of dihydroxy-substituted aromatic hydrocarbon in the second portion can be difficult to determine accurately and consistently.
Accordingly there is a need in the art for an improved method of controlling the molecular weight of a polymer over a range of molecular weights.